Hereinafter, a sharpened needle-like electrode which generates electrons or ions in an electron source of an electron microscope or a gas field ion source (GFIS) of a focused ion beam (FIB) apparatus, a probe in a scanning probe microscope (SPM), or the like is referred to as a “tip”.
Conventionally, in order to obtain a high resolution image in an electron microscope and a focused ion beam apparatus, the apex of a tip has been desired to be sharpened in a level of several atoms. Further, in order to prolong the lifetime of a tip in a scanning probe microscope, the apex of a tip has been desired to be sharpened to an atomic level.
FIGS. 15A to 15C show a schematic shape of a conventional tip 500. As shown in FIG. 15A, the tip 500 is formed such that the tip end of a fine wire having a diameter of several hundreds of micrometers or less has a narrow and sharp shape by electrolytic polishing (also referred to as “wet etching”). As shown in FIG. 15B, the tip 500 has a minute protrusion 501 in the apex portion B. As shown in FIG. 15C, the protrusion 501 has a triangular pyramid shape formed by several atomic layers, and the apex of the protrusion 501 is constituted by a single atom. Hereinafter, the protrusion 501 is referred to as a “pyramid structure”.
Incidentally, there has been known an ion microscope which includes a gas field ion source using a tungsten tip. In the tip, a crystal plane with a low planar atomic density tends to be sharpened, and therefore, the tungsten tip is sharpened in the <111> direction. The {111} crystal plane of tungsten has a threefold rotational symmetry, and a {110} crystal plane or a {112} crystal plane composes a side surface (pyramid surface) of a pyramid structure.
As a method for sharpening the apex of the tungsten tip in a level of several atoms, there has been known a method such as field-induced gas etching using nitrogen or oxygen, thermal faceting, or remolding, and by those methods, the apex in the <111> direction can be sharpened with high reproducibility.
The field-induced gas etching is a method for etching a tungsten tip by introducing nitrogen gas while observing a Field Ion Microscope (FIM) image using helium or the like as an imaging gas in an FIM. The ionization field strength of nitrogen is lower than that of helium, and therefore, nitrogen gas cannot come closer to a region where the FIM image can be observed (that is, a region where helium is field-ionized) and is adsorbed on the side surface of the tip at a short distance away from the apex of the tungsten tip. Then, the nitrogen gas is bonded to a tungsten atom on the surface of the tip to form tungsten nitride. Tungsten nitride has low evaporation field strength, and therefore, only the side surface of the tip at a short distance away from the apex on which nitrogen gas is adsorbed is selectively etched. At this time, a tungsten atom at the apex of the tungsten tip is not etched, and therefore, a tip having a further sharpened apex than an electrolytically polished tip can be obtained (for example, U.S. Pat. No. 7,431,856B).
The thermal faceting is a method for forming a polyhedral structure at the apex of a tip by heating the tip after electrolytic polishing in an oxygen atmosphere to grow a specific crystal plane (for example, JP2009-107105A).
The remolding is a method for forming a crystal plane at the apex of a tip by heating and applying a high voltage to the tip after electrolytic polishing under ultrahigh vacuum conditions (for example, JP2008-239376A).
Further, as a method for forming a tip having a structure in which the apex of the tip is constituted by a single atom, there has been known a method in which the surface of a tungsten or molybdenum tip is plated with gold, platinum, palladium, iridium, rhodium, or an alloy thereof, and then, the tip is terminated with a single atom by electrolytic polishing or heating (for example, JP2006-189276A).
Further, there has been known a scanning ion microscope (i.e. a focused ion beam apparatus) using a focused helium ion beam which includes a gas field ion source using a tungsten tip (for example, William B. Thompson et al., Proceedings of the 28th Annual LSI Testing Symposium (LSITS 2008), (2008) pp. 249-254, “Helium Ion Microscope for Semiconductor Device Imaging and Failure Analysis Applications”).
Further, it has been known that in a scanning ion microscope using a focused helium ion beam which includes a gas field ion source using a tungsten tip, the apex of the tungsten tip which emits ions is terminated with a trimer composed of three tungsten atoms (for example, B. W. Ward et al., Journal of Vacuum Science & Technology, vol. 24, (2006), pp. 2871-2874, “Helium ion microscope: A new tool for nanoscale microscopy and metrology”).
Further, it has been known that in order to form a tip having a pyramid structure in which the apex of a fine wire is constituted by a single atom with iridium having higher chemical resistance than tungsten as a material of the tip, it is essential to perform heating (thermal faceting) while introducing oxygen into a vacuum vessel (for example, JP2009-107105A).
Further, it has been known that a minute triangular pyramid structure composed of one {110} crystal plane and two {311} crystal planes is formed at the apex of a <210> iridium single crystal tip (for example, Ivan Ermanoski et al., Surf. Sci. vol. 596, (2005), pp. 89-97, “Atomic structure of O/Ir (210) nanofacets”).
Further, it has been known that a minute triangular pyramid composed of one {110} crystal plane and two {311} crystal planes is formed by thermal faceting at the apex of a sharpened <210> iridium single crystal tip, and the apex thereof is constituted by a single atom. A gas field ion source using this iridium tip can continuously emit a beam for about 2,250 seconds (for example, Hong-Shi Kuo et al., Nanotechnology, vol. 20, (2009) No. 335701, “A Single-atom sharp iridium tip as an emitter of gas field ion sources”).